The present invention relates in general to soft handoffs and in particular to methods and systems for establishing soft handoffs between two base stations (BSs) using a direct BS to BS link.
When a mobile station (MS), in a cellular communication system, crosses from one cell to another adjacent cell a process designated as handoff is initiated.
Handoff is the process by which a new air interface channel between a mobile station and a base station is established.
There are several types of handoff in the wireless communication industry. A xe2x80x9chard handoffxe2x80x9d is a handoff which requires a mobile station to tune its radio equipment or to reestablish synchronization. A xe2x80x9csoft handoffxe2x80x9d is a handoff that does not require the mobile station to tune its radio equipment or to reestablish synchronization and that uses the same frame selection function for (and voice trans-coding function, if this is a voice call) in the network for both the old and new air interface channels. Other types of handoffs, such as a semi soft handoff, are not pertinent to an understanding of the present invention. At the present time, only code division multiple access (CDMA) wireless communication systems are capable of accomplishing soft handoffs.
In a CDMA wireless communication system, using cellular transmission technology, the process of establishing a soft handoff from the time an MS sees a new pilot signal, received from a nearby base transceiver station (BTS) defining another cell, to the time an MS sends a handoff completion message can be segregated into at least three phases.
Phase 1 may be defined as occurring from the time the MS sees a new pilot to the time that the pilot strength exceeds a predetermined threshold and may be referred to as the defection phase. Phase 2 may be defined as occurring between the time an MS sends a pilot strength measurement (PSM) message until it receives a handoff direction message and may be referred to as the establishing phase (when successful). Phase 3 may be defined as the time from when an MS receives a handoff direction message to the time the MS sends a handoff completion message and may be referred to as the completion phase.
As is well-known in the art, many standards are used by the communication industry to define functional standards by which communication equipment will operate so that equipment from various manufacturers may compatibly inter-operate. One such standard is IS-95 Revision A entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems; May, 1995xe2x80x9d. Another standard is IS-634 Rev A ballot version dated Apr. 20, 1997 entitled xe2x80x9cMSC BS Interface for Public Wireless Communicationxe2x80x9d that among other items defines an xe2x80x9cA3xe2x80x9d communication link between base station entities. A further standard mentioned in this document is IS-41-C entitled xe2x80x9cCellular Radiotelecommunications Intersystem Operations; 1996xe2x80x9d. Each of these standards are incorporated herein by reference.
The IS-634 standard has defined and standardized communications between a mobile switching center (MSC) and a BS by industry agreement such that different manufacturers equipment could be combined into a composite cellular system by a user. However this standard has not previously attempted to standardize any signaling messages between base stations. Thus, signaling messages bound from a source base station to a target base station, must be first relayed to an MSC in a standardized manner and protocol before being received by the target base station.
To support soft handoff of an MS moving from one cell to another, adjacent cells must operate at the same frequency. Once an MS detects a new pilot, the MS may experience same frequency interference from adjacent cells. In accordance with the standards of IS-95, 12 consecutive bad frames detected by an MS may cause the MS to drop a call. The total network processing time that may cause a call to be dropped is primarily a function of the phase 2 time period. At this time the received signal has already degraded enough that a handoff has been requested. This typically means that the MS is still moving away from its signal source. Thus the degradation of received signal quality, before handoff completion, may result in the occurrence of 12 consecutive bad frames as detected by the MS.
One prior art system used 12 steps and including the transmission of 12 messages for an intra-system inter-BS handoff during phase 2. The series of setup and request messages went from the MS, to the serving BTS, to the selection/distribution unit (SDU), the serving base station controller (BSC), the mobile switching center (MSC), the target BSC, and the target BTS. The response messages came from the target BTS to the target BSC, the MSC, the serving BSC, the SDU and finally the serving BTS.
For an inter-system handoff, 14 steps were completed because of the extra MSC involved in both directions.
It takes time to successfully transmit a message and to process the message after receipt. It will be apparent that the time required to complete the referenced 12 steps must occur before the MS has moved to a position such that the quality of the signal received by the MS degrades to a point whereby the MS detects 12 consecutive bad frames. This is especially true when the MSC is involved in many administrative tasks other than call processing.
Thus a system whereby the number of steps involved and the number of messages transmitted could be reduced for the establishment process of phase 2 would lessen the chances of a call being dropped due to extensive time required to complete this phase. The elimination of the MSC from the message path, even where there was no reduction in number of steps involved, due to a required addition of dedicated processing entities, would also reduce the time required to complete phase 2 in some circumstances.
It would further be desirable, if base stations, whether identical or made by different manufacturers, could communicate directly with one another to establish a soft handoff of an MS across cellular boundaries of communication cells managed by different base stations.
The present invention comprises providing an enhanced communication link directly between source and target base stations. This enhanced link permits the transmission of both signaling and voice information thereby removing the involvement of the MSC from the soft handoff establishing phase process.